CDL COMBINATION VEHICLES PRACTICE
TEST QUESTIONS AND CORRECT
ANSWERS (VERIFIED ANSWERS) PLUS
RATIONALES 2025
1. What is the purpose of the service air line?
A. To supply air to the trailer’s emergency brakes
B. To supply air to the trailer air tanks
C. To carry air to the trailer brakes
D. To control the parking brake
The service air line carries air that is controlled by the foot brake or trailer
hand brake to apply the trailer service brakes.
2. What does the emergency air line do?
A. Provides signal for trailer lights
B. Supplies air to the trailer tanks and controls emergency brakes
,C. Activates the engine shutdown system
D. Releases the parking brake on the tractor
The emergency line supplies air to the trailer air tanks and engages the
emergency brakes if the line breaks or loses pressure.
3. If the air lines cross when hooking up to an older trailer, what will
happen?
A. The trailer will not brake properly
B. The air compressor will be damaged
C. The trailer spring brakes may not release
D. Nothing, the system corrects itself
Crossed lines can prevent the trailer’s spring brakes from releasing, making
it unsafe to operate.
4. When inspecting the coupling, where should the locking jaws be?
A. Closed around the kingpin
B. Open and clear
C. Locked around the shank of the kingpin
D. Pressed against the trailer frame
The locking jaws should be secured around the shank of the kingpin,
ensuring the trailer is properly coupled.
, 5. When should the trailer hand valve be used?
A. To park the trailer
B. To test trailer brakes only
C. During highway driving
D. To apply brakes to both tractor and trailer
The trailer hand valve is used to test the trailer brakes separately and
should not be used while driving.
6. What is the function of the fifth wheel?
A. Controls the air suspension
B. Connects the tractor to the trailer
C. Applies the emergency brake
D. Levels the trailer
The fifth wheel securely connects the tractor and trailer, allowing
articulation.
7. What happens when the trailer is not high enough when backing
under it?
A. The trailer can push the tractor off its path
B. It connects more securely
, C. Nothing unusual
D. The tractor brakes lock
If the trailer is too low, it can damage the tractor or cause misalignment.
8. What is a tractor protection valve?
A. A backup brake system
B. An engine safety device
C. A valve that shuts off air to the trailer to prevent loss
D. A valve that controls hydraulic pressure
This valve protects the tractor air supply from major leaks in the trailer.
9. To prevent a rollover when pulling doubles, you should:
A. Speed up through turns
B. Steer gently and brake smoothly
C. Use the hand valve only
D. Apply brakes hard
Smooth braking and steering reduce the risk of rollovers, especially with
combination vehicles.
10. How should you test that air flows to all trailers?
TEST QUESTIONS AND CORRECT
ANSWERS (VERIFIED ANSWERS) PLUS
RATIONALES 2025
1. What is the purpose of the service air line?
A. To supply air to the trailer’s emergency brakes
B. To supply air to the trailer air tanks
C. To carry air to the trailer brakes
D. To control the parking brake
The service air line carries air that is controlled by the foot brake or trailer
hand brake to apply the trailer service brakes.
2. What does the emergency air line do?
A. Provides signal for trailer lights
B. Supplies air to the trailer tanks and controls emergency brakes
,C. Activates the engine shutdown system
D. Releases the parking brake on the tractor
The emergency line supplies air to the trailer air tanks and engages the
emergency brakes if the line breaks or loses pressure.
3. If the air lines cross when hooking up to an older trailer, what will
happen?
A. The trailer will not brake properly
B. The air compressor will be damaged
C. The trailer spring brakes may not release
D. Nothing, the system corrects itself
Crossed lines can prevent the trailer’s spring brakes from releasing, making
it unsafe to operate.
4. When inspecting the coupling, where should the locking jaws be?
A. Closed around the kingpin
B. Open and clear
C. Locked around the shank of the kingpin
D. Pressed against the trailer frame
The locking jaws should be secured around the shank of the kingpin,
ensuring the trailer is properly coupled.
, 5. When should the trailer hand valve be used?
A. To park the trailer
B. To test trailer brakes only
C. During highway driving
D. To apply brakes to both tractor and trailer
The trailer hand valve is used to test the trailer brakes separately and
should not be used while driving.
6. What is the function of the fifth wheel?
A. Controls the air suspension
B. Connects the tractor to the trailer
C. Applies the emergency brake
D. Levels the trailer
The fifth wheel securely connects the tractor and trailer, allowing
articulation.
7. What happens when the trailer is not high enough when backing
under it?
A. The trailer can push the tractor off its path
B. It connects more securely
, C. Nothing unusual
D. The tractor brakes lock
If the trailer is too low, it can damage the tractor or cause misalignment.
8. What is a tractor protection valve?
A. A backup brake system
B. An engine safety device
C. A valve that shuts off air to the trailer to prevent loss
D. A valve that controls hydraulic pressure
This valve protects the tractor air supply from major leaks in the trailer.
9. To prevent a rollover when pulling doubles, you should:
A. Speed up through turns
B. Steer gently and brake smoothly
C. Use the hand valve only
D. Apply brakes hard
Smooth braking and steering reduce the risk of rollovers, especially with
combination vehicles.
10. How should you test that air flows to all trailers?
